The present invention relates to a riser assembly and method of providing a riser assembly. In particular, but not exclusively, the present invention relates to a riser assembly suitable for use in the oil and gas industry, to help prevent unwanted movement of buoyancy modules after installation.
Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 meters or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.
In known flexible pipe design the pipe includes one or more tensile armour layers. The primary load on such a layer is tension. In high pressure applications, the tensile armour layer experiences high tension loads from the internal pressure end cap load as well as weight. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time.
One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. The buoyancy aids provide an upwards lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time. Examples of known riser configurations using buoyancy aids to support the riser's middle section are shown in FIGS. 1a and 1b, which show the ‘steep wave’ configuration and the ‘lazy wave’ configuration, respectively. In these configurations, there is provided a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a subsea location to a floating facility 202 such as a platform or buoy or ship. The riser is provided as a flexible riser, i.e. including a flexible pipe, and includes discrete buoyancy modules 204 affixed thereto. The positioning of the buoyancy modules and flexible pipe can be arranged to give a steep wave configuration 2061 or a lazy wave configuration 2062.
Wave riser configurations as shown in FIGS. 1a and 1b are often used in shallow water applications so as to allow for excursions of the vessel from the point where the riser contacts the sea bed.
However, in some applications, the buoyancy modules may react to changes in riser assembly weight, for example caused by marine growth (shellfish and other sea life and/or sea debris attaching to the riser). Alternatively or additionally, the riser assembly and/or the buoyancy modules may experience a gradual (or sudden) change in content density due to movement or general day to day wear for example. This may cause the amount of buoyancy support or net buoyancy (and therefore the relative height above the sea bed) of the riser to change. Any change in the amount of buoyancy support may have an adverse effect on the tension relief provided to the flexible pipe, which could ultimately decrease the lifetime of a riser.
Furthermore, such changes in weight could lead to an undesirable situation where the riser assembly diverts completely from its designated configuration by either popping up to the water's surface or sinking to the sea bed. This is particularly applicable to shallow water applications (less than 1000 feet (304.8 meters)), since any change in buoyancy has a more pronounced effect on the height change at shallow depths. Interference with any neighbouring riser assemblies or vessel structures could become a problem.
WO2009/063163, incorporated herein by reference, discloses a flexible pipe including weight chains secured to a number of buoyancy modules on the pipe. The chains hang from the buoyancy modules, extending downwards to the sea bed and having an end portion lying on the sea bed. The weight associated with each length of chain counteracts the buoyancy provided by the respective buoyancy module to which it is secured. That is, when the density of a riser section decreases and the pipe begins to rise towards the water surface, the amount of chain suspended between the buoyancy module and the seabed is increased (i.e. heavier), thus offsetting the pipe's tendency to rise upwards. When the density of a riser section increases and the pipe begins to descend towards the sea bed, the amount of chain suspended between the buoyancy module and the seabed is decreased (i.e. lighter), again offsetting the pipe's tendency to descend down to the sea bed.
It would be useful to provide an alternative to the assembly described in WO2009/063163.
In addition, particularly in shallow water applications, it would be useful to be able to ensure a certain minimum clearance distance between a so-called sag bend and the seabed, and also between a so-called hog bend and the sea surface and/or a vessel or structure at the surface. A sag bend 208 is a U-shaped bend in a riser and a hog bend 210 is an inverted U-shaped bend in a riser (as indicated in FIG. 1a). This would help to ensure that the riser does not pop up to the water's surface or sink to the sea bed, or collide with a vessel or other structure.